Windows Sandbox broken in Insider build 18305.1003

A small cumulative update has broken one of Windows 10 preview build 18305's biggest features.

On December 20, Microsoft shipped an Internet Explorer security fix to the latest Windows 10 Fast ring Insider build, bringing it up to build number 18305.1003. In an update to the build's list of known issues, Microsoft this week confirmed that the fix also had the effect of breaking both Windows Defender Application Guard and Windows Sandbox (via OnMSFT).

Build 18305 brought with it a number of interesting new features, but Windows Sandbox is potentially one of the biggest surprises. The feature allows you to fire up a clean, virtual desktop environment through which you can run and test untrusted software without worrying about its impact on your base installation. Once Windows Sandbox is closed, all of its files and software are permanently deleted.